Recently, it has been studied to develop various devices for light modulation by forming an optical waveguide in various kinds of materials. Such devices are to be used in systems for optical communication, a projector etc. Particularly, so-called ridge shaped optical waveguides can improve the confinement of light to provide a high efficiency of controlling light. The waveguide structure has been thus expected as optical modulators, optical switching devices, wavelength converting devices, or the like.
Non-linear optical crystals, such as lithium niobate or lithium tantalite single crystal, have a high second-order non-linear optical constant. It is thus possible to realize a quasi-phase matched (QPM) type second harmonic generation (SHG) device, by providing a periodic polarization domain inversion structure in the crystal. Further, an SHG device of a high efficiency can be realized by forming a waveguide in the periodic polarization domain inversion structure, and the device finds a wide variety of applications such as optical communication, displays, medical uses, opt-chemical uses, various optical measurements, etc.
FIG. 1 shows a structure of forming a channel type optical waveguide in a ridge portion. A layer 3, made of MgO-doped lithium niobate for example and a supporting body 1 are adhered through an adhesive layer 2. A pair of elongate grooves 6, parallel with each other, are formed in the layer 3, so that the grooves 6 form a ridge portion 5. The ridge portion 5 and grooves 6 together form an optical waveguide structure 4, so that the ridge portion 5 functions as a three-dimensional or channel type optical waveguide. According to such optical waveguide structure, it is normally difficult to make side faces 5b and 8 facing the groove 6 perpendicular to an upper face 5a and they are inclined with respect to an axis perpendicular to the upper face 5a. Further, a bottom face 7 of the groove 6 is substantially flat.
However, in the lower side of the bottom face 7, incident light (fundamental wave in the case of a harmonic wave generating device) tends to be leaked as arrow A, so that it becomes difficult to propagate and confine the light in the ridge portion. Further, during the process of forming the ridge portion 5, tipping tends to occur at both edges of the ridge portion 5, resulting in an increase of its optical propagation loss.
In WO 2006/041172A1, the inventors disclosed that a channel type optical waveguide is formed in a ridge portion, protuberances are formed on both sides of the ridge portion, respectively, and grooves are formed between the respective protuberances and ridge portion. It is thereby tried to reduce the tipping at the both edges of the ridge portion and to strengthen the confinement of light in lateral direction to improve the propagation efficiency.